Standard R incandescent lamp has a medium screw base.
In an incandescent lamp, light is generated by heating the filament to incandescence. The hotter the filament, the more efficient it is in converting electricity to light. However, when the filament operates hotter, its life is shortened so the design of each lamp is a balance between efficiency and life. This is why lamps of equal wattage may have different lumen ratings and different life ratings. Incandescent lamps of similar size are commonly available with different wattage ratings. The fixture wattage limit should not be exceeded.